1. Field of the Invention
This invention relates to an improvement in oil refining process. In particular, this invention relates to a method to improve remarkably the yield of lighter components in a refining process of heavy oils without any necessity to change details of the existing oil refinery plants.
This invention relates also to an additive for increasing the yield of lighter components in the oil refinery process.
This invention also relates to a novel use of a compound (I) having at least one mercaprtoalkylthio group: HS--C.sub.m H.sub.2m --S--, in which "m" is an integer of 2 to 4.
2. Description of Related Art
One of objects of oil refinery is to increase the yield of lighter components such as kerosene and gas oil which are much value added products comparing to heavier components which are less value added products. In fact, in actual oil refinery processes, heavier components such as residual oil from atmospheric distillation column is heat-treated in vacuum distillation column, thermal cracking unit or visbreaking unit to obtain lighter components. These processes and apparatuses are well-known and are described in many books, for example "World Encyclopedia" 13, Heibon-sha, 1966, p 237-254.
The yield of lighter components can be increased by elevating operation temperature in the heating units. However, elevation of operation temperature in the heating units cause a trouble of increment of cokes which deposit on inner walls of heating unit or heat-exchangers, resulting in frequent stop of production plants for maintenance. In particular, in a case of thermal cracking process of heavy oil, heating tubes in a heating furnace are seriously contaminated and are choked with deposited cokes, so that production plant can not be operated continuously for longer time but must be stopped frequently for cleaning.
Due to this problem, actual oil refinery plants are compelled to be operated at relatively lower operation temperature than such ideal operation temperature that assure higher yield of lighter components. In other words, operation temperature can not be elevated above certain limit.
Inventors found, surprisingly, such a fact that the yield of lighter components can be increased remarkably by incorporating an additive having specific mercapto alkylthio group in material of heavy oil to be refined in heating units used in a variety of oil refining processes, and completed the invention.